trans-pacific bathymetry survey crossing over the …€¦ · spreading ridge axis and defined by...
TRANSCRIPT
43
JAMSTEC Rep. Res. Dev., Volume 17, September 2013, 43_57
— Report —
We carried out underway geophysical survey in the transit of the JAMSTEC R/V Mirai MR08-06 Leg-1. The cruise was an
unprecedented opportunity to collect data in regions of the Pacific Ocean where it has sparsely been surveyed. Our multibeam bathymetric
and shipboard gravity survey track crossed over the Pacific, the Antarctic, and the Nazca plates, and covered lithospheric ages varying
from zero to 150 Ma. The survey revealed kilometer-sized fine-scale structures of seafloor fabrics; i.e. abyssal hills and fracture zones, and
distribution of seamounts or knolls. These are not detectable in satellite altimetry data only. As well as contributing to the world's seafloor
mapping, our survey results also show valuable evidence towards the plate tectonic reconstruction and help us look into the oceanic
lithosphere formation and evolution, since the directions of tectonic stress and seafloor spreading mode are the major factors that can affect
the morphology of lineated abyssal hills, etc.
Keywords: bathymetry, gravity anomaly, Pacific plate, Antarctic plate, Nazca plate, abyssal hill, fracture zone, age-depth relationship
Received 2 April 2013 ; Revised 17 June 2013 ; Accepted 17 June 2013
1 Institute for Research on Earth Evolution (IFREE), Japan Agency for Marine-Earth Science and Technology (JAMSTEC)
2 Observation and Research Department, Global Ocean Development Inc.
*Corresponding author:
Natsue Abe
Institute for Research on Earth Evolution (IFREE), Japan Agency for Marine-Earth Science and Technology (JAMSTEC)
2-15 Natsushima-cho, Yokosuka, Kanagawa 237-0061, JAPAN
Tel. +81-46-867-9329
Copyright by Japan Agency for Marine-Earth Science and Technology
Trans-Pacific Bathymetry Survey crossing over the Pacific, Antarctic, and Nazca plates
Natsue Abe1*, Toshiya Fujiwara1, Ryo Kimura2, Asuka Mori2, Ryo Ohyama2, Satoshi Okumura2, and Wataru Tokunaga2
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1. Introduction
Multibeam bathymetric data reveal seafloor fabrics, i.e.
abyssal hills and fracture zones, distribution of seamounts and/or
knolls, for us to discuss the formation and evolution of the oceanic
lithosphere. The seafloor depths often indicate the structure of
oceanic lithosphere, thermal state, and mantle dynamics. Shipboard
gravity data, when combined with multibeam bathymetry, become
more accurate set of data to estimate fine-scale crustal structures and
subsurface mass distribution. The results can reveal features that are
usually smaller than several kilometers in width, which could not be
detected by global predicted bathymetry, or the conventional gravity
data derived from satellite altimetry. In this paper, we report on one long
survey line in the Pacific that crosses from the northeast Japan coast
through to the equator at the mid-Pacific on to the southwest Chilean
coast. Even if it is only one survey line, it shows several important
features in the non-survey areas, especially at the southeastern Pacific
area where the tectonics has not been well-defined.
The JAMSTEC R/V Mirai MR08-06 Leg-1 cruise
was conducted in January - March 2009 as a part of SORA2009
(Cruise data and reports; Abe, 2009; Harada, 2009) for geological
and geophysical studies in the southern Pacific (e.g. Suetsugu et
al., 2012; Anma and Orihashi, 2013). We carried out underway
geophysical survey in the transit. The MR08-06 Leg-1 cruise was
an unprecedented opportunity to collect data in the regions of the
Pacific Ocean where it has been sparsely surveyed using state-of-
the-art echo-sounding technology. Here we present the character of
our trackline geophysical data.
2. Data Acquisition
The MR08-06 Leg-1 cruise started on 15 January 2009,
Sekinehama Japan, stopped by at Papete Tahiti during 3-6 February
on the way to Valparaiso Chile, and the cruise ended on 14 March
2009 (Abe, 2009).
Fig. 1. (a) Index map of the survey. The bathymetric data are from ETOPO1 1 arc-minute global model (Amante and Eakins, 2009). The red line shows the track of
the R/V Mirai MR08-06. Yellow crosses indicate locations of XBT and XCTD observations.
180˚ 120˚W 60˚W
30˚S
0˚
30˚N
-10000 -8000 -6000 -4000 -2000 0 2000 4000 6000 8000 10000 m
180˚ 120˚W 60˚W
30˚S
0˚
30˚N
Sekinehama
Papete
Valparaiso
(a)
Antarctic Plate
Nazca Plate
Pacific Plate
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Swath bathymetric data were obtained using a SeaBeam
2112 multi-narrow beam echo sounder system with a 12 kHz
frequency and a 2°×2° beam width. A swath width was set to 120°,
covering an across-track width triple as wide as the water depth.
Sound velocity profiles in the water column were calculated using
measurements from XBT at 6 sites and XCTD at 8 sites (Figure
1(a)). The survey ship's speed during the survey was 14-15 kt.
Marine gravity field was measured using a LaCoste and Romberg
air-sea gravity meter S-116. Shipboard gravity data were tied to
absolute gravity values at calibration stations in Sekinehama,
Papete, and Valparaiso. The sensor drift rate was 0.056 mGal/
day. Marine geomagnetic field was also measured using a three-
component magnetometer permanently installed on the ship's deck.
For results of the magnetic anomaly, refer to other papers (Kise et
al., 2010; Matsumoto et al., 2013).
After the transit cruise, the ~22000 km long trans-Pacific
track, traveling halfway around the globe, was completed. The
Pacific, the Antarctic, and the Nazca plates were crossed over, and
lithospheric ages vary from zero to 150 Ma (Figure 1(b)).
3. Results and Discussion
3.1. Basement DepthsAlong ship track profiles of observed bathymetry are
shown in Figure 2. The bathymetry was corrected for isostatic
effects due to sediment load. Sediment correction was calculated
by using Schroeder's method (1984). The compiled data of
sediment thickness were given by Divins (2003). Lithospheric age
along the track was sampled from the digital data of Müller et al.
(2008). Theoretical depth models, as a function of corresponding
lithospheric age, are from Parsons and Sclater (1977) (PS), and
Fig. 1. (b) Lithospheric age in Ma (Müller et al., 2008). The isochrons are the same chrons as those used by Müller et al. (2008), namely chrons 5o (10.9 Ma), 6o
(20.1 Ma), 13y (33.1 Ma), 18o (40.1 Ma), 21o (47.9 Ma), 25y (55.9 Ma), 31y (67.7 Ma), 34y (83.5 Ma), M0 (120.4 Ma), M4 (126.7 Ma), M10 (131.9 Ma), M16
(139.6 Ma), M21 (147.7 Ma), and M25 (154.3 Ma). Plate boundaries, magnetic lineations (Cande et al., 1989), fracture zones, and distinct topographic lineaments
are drawn on the map by solid lines.
180˚ 120˚W 60˚W
30˚S
0˚
30˚N
0.0
10.9
20.1
33.1
40.1
47.9
55.9
67.7
83.5
120.
412
6.7
131.
913
9.6
147.
715
4.3
180.
0
180˚ 120˚W 60˚W
30˚S
0˚
30˚N
Ma
(b)
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Trans-Pacific bathymetry survey
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Stein and Stein (1992) (GDH1).
As far as the mid-Pacific (~170°W) from the Pacific
Antarctic Ridge (PAR), the GDH1 model, based on a plate model
with a plate thickness of 95 km, a bottom boundary temperature of
1450°C, is consistent with the observation (Figure 2(a)). However,
the seafloor depth in the northwestern and old Pacific of the survey
area (150°E~170°E) is deep. The PS model (1350°C at the base
of a 125-km-thick plate) is consistent with the observation in this
area rather than the GDH1 model. Or long-wavelength free-air
gravity anomaly, which indicates isostatic anomaly, shows negative
values (Figure 2(b)). That suggests the lithosphere is dynamically
depressed. Adam and Vidal (2010) proposed a relationship between
depth and distance from a mid-ocean ridge along a mantle flow-
line of the Pacific Plate motion. The model fits this depth profile
in the old Pacific and is generally applicable to the profile (Figure
2(a)). However the model predicts a depth somewhat shallower in
the younger seafloor. Although more work is needed to evaluate the
model, we may have to consider lithospheric cooling with age as
well.
3.2. Seafloor FabricsObserved bathymetric swath and tectonic circumambient
are shown in Figure 3, and detailed maps of the swath in some
notable areas are shown in Figure 4. The bathymetry revealed
fine-scale structure of seafloor fabrics; the sizes of the structure
are smaller than several kilometers and had never been revealed in
global predicted bathymetry like ETOPO1 (Amante and Eakins,
2009). The difference is the manifest in the sparsely surveyed
southern Pacific Ocean (See Figures 4(e) and 4(f), Figures 4(h) and
4(i) for comparison). The seafloor fabrics mainly originate from a
mid-ocean ridge system, where the oceanic lithosphere was formed.
The lineated abyssal hills are the consequence of seafloor spreading
and succeeding normal faulting. Transform faulting sculpts a
fracture zone perpendicular to the abyssal hills. Consecutive trends
of lineated abyssal hills and fracture zones indicate stable tectonic
stress field originated from the PAR (Figures 3(f) and 4(g)) and
the Chile Ridge spreading systems (Figures 3(h) and 4(l)). The
ridge axis of the PAR located at 113°20'W is typical of a fast-
spreading ridge axis and defined by rise topography over a broad
cross section (Figures 2 and 4(g)). The seafloor fabric morphology
revealed a clear boundary between the PAR and the Chile Ridge
domains (Figures 3(g) and 4(h)). Crust formed at the PAR and at
the Chile Ridge are separated there. Azimuths of the seafloor fabric
change from 5°, which is sub-parallel to the PAR axis's strike, to
100° at 95°00'W. Previous studies predicted a trace of the Pacific-
Antarctic-Farallon (Nazca) plates' triple junction (e.g. Tebbens et
al., 1997). Probably the observed bathymetric boundary is a part of
the trace. The result will be constraint for future studies of the plate
reconstruction and tectonic evolution of the PAR, the Chile Ridge,
and the Antarctic Plate.
Fluctuation of the seafloor fabric strikes suggests
instability of tectonic stress fields (Figures 4(d) and 4(k)). Especially
the strike of seafloor fabric varying from -40° to 50° (Figure 4(k))
may be largely influenced by the tectonic structure of offsets at
fracture zones system separated by short ridge segments. The
survey track lies near an intersection of the ridge segment and the
Taitao fracture zone, and the offset length is shorter there (Figure
3(h)). At the fracture zone, the offset increases as the age decreases
due to ridge jumps (Bourgois et al., 2000) or change in spreading
rates (Matsumoto et al., 2013). This indicates the possibility of
some dominant stress affecting spatially and/or temporally, from
normal stress caused by seafloor spreading to shear stress caused by
strike-slip throughout the evolution at the fracture zone. In contrast,
abyssal hills elongated in the direction of -5° originated from the
Chile Ridge system and fracture zones having long offset lengths
distinctly bisect at right angles (Figure 4(l)). The regionality of the
seafloor morphology on the Chile Ridge flank was found (Figures
4(k) and 4(l)). The Morphology of faulting may yield differences in
seawater percolation into the crust and the uppermost mantle.
Non-transform offset or pseudo-faults formed by mid-
ocean ridge's propagation may cause shear stress field, which may
result in crustal deformation and curvilinear seafloor fabric (Figures
4(e) and 4(j)). The observed depression at 39°05'S shown in Figure
4(e) appears to be the Adventure Trough. The trough was presumed
to be a pseudo-fault formed by a southward propagating rift (Cande
and Haxby, 1991). Tectonics of the region shown in Figure 4(j) was
unconstrained by previous studies (e.g. Tebbens et al., 1997). The
found curvilinear seafloor fabric may become a clue to the tectonics.
The survey track passed through a suture zone formed
at a paleo-ridge-triple-junction (e.g. Nakanishi et al., 1992) (Figure
3(a)). Azimuth angles of abyssal hills vary from 0° to 40°, NE-SW
(Figure 4(a)). The area shown in Figure 4(a) is situated near east
of the magnetic Magellan Lineation Set identified by Nakanishi et
al. (1992) (Figure 3(a)). The lineation trends the NW-SE direction,
and is thus different from that in the Figure 4(a) area. Such a short-
range directional change may be due to complex tectonic evolution
around the triple junction as suggested by Nakanishi and Winterer
(1998).
Magnetic lineations are unconstrained on the seafloor
in the Cretaceous magnetic quiet (125-80 Ma) zone (Figures 1(b),
3(b), and 3(c)). Thus, strikes of lineated abyssal hills give critical
evidence for future studies of the plate reconstruction and tectonic
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JAMSTEC Rep. Res. Dev., Volume 17, September 2013, 43_57
evolution of the old Pacific Plate (Figures 4(b) and 4(c)). The
azimuth of the fabric is found to be 0° although small volcanic
knolls are overprinted on the lineated seafloor fabric of abyssal hills.
We also detected many small seamounts and knolls
superimposed on the seafloor fabrics. These are considered to
be constructed by excess magmatism at a mid-ocean ridge or
intra-plate volcanism. The seamounts and knolls distributions are
discussed in other papers (Hirano et al., 2013a, b). Our data will
be a useful contribution for the global distribution of intra-plate
volcanism that produces small knolls such as the petit-spot (e.g.
Hirano et al., 2006).
Additional surveys may be needed for future in-depth
studies, however our survey gives some valuable and suggestive
evidence for the plate tectonic reconstruction and studies on
formation and evolution of the oceanic lithosphere.
-8000
-6000
-4000
-2000
0
-200
-100
0
100
200
150E 180 150W 120W 90W
150
100
50
0
Longitude (°)
Dep
th (m
)Ag
e (M
a)F.G
.A. (mG
al)
Japa
n Tr
ench Shatsky Rise M. Pac. M. Tahiti P. A. R. Chile Ridge
DepthGDH1
AV
Age
FGA
PSBMT
(a)
(b)
Fig. 2. Profiles along the ship track. (a) Observed water depth (Depth), corrected basement depth (BMT), seafloor depth models from Parsons and Sclater (PS),
Stein and Stein (GDH1), and Adam and Vidal (AV). (b) Lithospheric age (Age) and free-air gravity anomaly (FGA).
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(a)
(b)
Fig. 3. Bathymetric swath illuminated from northwest superimposed on ETOPO1. The color map along the track was made using inner beams within swath
angle of 90° . Note that a different color scale is used in each figure. Green line contours indicate lithospheric age from Müller et al. (2008). Fracture zones and
topographic lineaments are shown in dark blue lines and reported magnetic lineations are shown in light blue lines. Magnetic anomaly numbers are attached on some
selected lines. These are from the same data set as shown in Figure 1(b). Distinct strikes of seafloor fabrics and features detected by using the fine-scale bathymetry
are labeled with red-color letters along the track.
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(c)
(d)
Fig. 3. (Continued)
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(e)
(f)
Fig. 3. (Continued)
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(g)
(h)
Fig. 3. (Continued)
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Trans-Pacific bathymetry survey
JAMSTEC Rep. Res. Dev., Volume 17, September 2013, 43_57
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m
m
m
m
0° 30°60°
90°
0° 30° 60°
90°
0°30
° 60°
90°
0° 30° 60°
90°
Fig. 4. Detailed maps of the bathymetric swath. Locations of these maps are marked in Figure 3. Note that a different color scale is used in each figure. The
alphabetical order is arranged from the northwest (a) to the southeast (l). (a) Azimuth angles of abyssal hills vary from 0° (left in the figure) to 40° (right).
(b), (c) Small knolls are superimposed on a lineated seafloor fabric of abyssal hills of the Cretaceous seafloor. (d) Fluctuation of strike of the seafloor fabric
between -10° and 10°.
(a)
(b)
(c)
(d)
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131˚00'W
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30 km
-3000 -2500 -2000 m
(g)
0°30°60°90°
0°30
° 60°
90°
Fig. 4. (Continued)
(e) Curvilinear seafloor fabric at the intersection area of the Agassiz Fracture Zone and the Adventure Trough. (f) ETOPO1 bathymetry of the same area of (e) for
comparison. (g) An axial ridge of the Pacific Antarctic Ridge (PAR) is situated at 113°20'W. Consecutive lineated abyssal hills trend 5°.
(e)
(f)
(g)
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96˚20'W
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0°30°60°90°
Fig. 4. (Continued)
(h) Boundary between the PAR and the Chile Ridge domains. (i) ETOPO1 bathymetry of the same area of (h) for comparison. (j) Curvilinear seafloor fabric in the
Chile Ridge domain.
(h)
(i)
(j)
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87˚2
0'W
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0'W
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86˚2
0'W
86˚0
0'W
85˚4
0'W
85˚2
0'W
85˚0
0'W
84˚4
0'W
48˚20'S 48˚20'S
48˚00'S
87˚2
0'W
87˚0
0'W
86˚4
0'W
86˚2
0'W
86˚0
0'W
85˚4
0'W
85˚2
0'W
85˚0
0'W
84˚4
0'W
48˚20'S 48˚20'S
48˚00'S
87˚2
0'W
87˚0
0'W
86˚4
0'W
86˚2
0'W
86˚0
0'W
85˚4
0'W
85˚2
0'W
85˚0
0'W
84˚4
0'W
48˚20'S 48˚20'S
48˚00'S30 km
-4500 -4000 -3500
79˚40
'W
79˚00
'W
78˚20
'W
78˚00
'W
77˚40
'W
77˚20
'W47˚40'S
47˚00'S
46˚40'S
46˚20'S
79˚40
'W
79˚00
'W
78˚20
'W
78˚00
'W
77˚40
'W
77˚20
'W47˚40'S
47˚00'S
46˚40'S
46˚20'S
79˚40
'W
79˚00
'W
78˚20
'W
78˚00
'W
77˚40
'W
77˚20
'W47˚40'S
47˚00'S
46˚40'S
46˚20'S
30 km
-4000 -3500 -3000 -2500
(k)
(l)
m
m
0° 30°
60°90
°
0°30°60°90°
Fig. 4. (Continued)
(k) The strike of seafloor fabric varying from -40° (left in the figure) to 50° (right). (l) Abyssal hills originated from the Chile Ridge system and fracture zones
perpendicular to the abyssal hills.
(k)
(l)
56
Trans-Pacific bathymetry survey
JAMSTEC Rep. Res. Dev., Volume 17, September 2013, 43_57
Acknowledgments
We express great thanks to the R/V Miraiʼs captain
Masaharu Akamine and the crew for their excellent operations.
The MR08-06 cruise was conducted with Co-Principal Dr. Naomi
Harada within a half-year cruise project SORA2009 (South
Pacific Ocean Research Activity 2009). We are grateful to the
shipboard scientific party for collaboration at sea and in scientific
discussions, and JAMSTEC data management office for help
with data processing. We thank Prof. Masao Nakanishi and an
anonymous reviewer for their helpful comments in improving the
manuscript. The GMT software (Wessel and Smith, 1991; 1995)
was extensively used in this study. Part of this work is a contribution
of the research program at the IFREE, JAMSTEC, and the Grant-in-
Aid for Scientific Research from the MEXT, Japan (No. 20340124).
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